Tin-coupled polymers are known to provide desirable properties, such as improved treadwear and reduced rolling resistance, when used in tire tread rubbers. Such tin-coupled rubbery polymers are typically made by coupling the rubbery polymer with a tin coupling agent at or near the end of the polymerization used in synthesizing the rubbery polymer. In the coupling process, live polymer chain ends react with the tin coupling agent thereby coupling the polymer. For instance, up to four live chain ends can react with tin tetrahalides, such as tin tetrachloride, thereby coupling the polymer chains together.
The coupling efficiency of the tin coupling agent is dependant on many factors, such as the quantity of live chain ends available for coupling and the quantity and type of polar modifier, if any, employed in the polymerization. For instance, tin coupling agents are generally not as effective in the presence of polar modifiers. The amount of coupling which is attained is also, of course, highly dependent upon the quantity of tin coupling agent employed.
Each tin tetrahalide molecule is capable of reacting with up to four live polymer chain ends. However, since perfect stoichiometry is difficult to attain, some of the tin halide molecules often react with less than four live polymer chain ends. For instance, if more than a stoichiometric amount of the tin halide coupling agent is employed, then there will be an insufficient quantity of live polymer chain ends to totally react with the tin halide molecules on a four to one basis. On the other hand, if less than a stoichiometric amount of the tin halide coupling agent is added, then there will be an excess of live polymer chain ends and some of the live chain ends will not be coupled.
Conventional tin coupling results in the formation of a coupled polymer which is essentially symmetrical. In other words, all of the polymer arms on the coupled polymer are of essentially the same chain length. All of the polymer arms in such conventional tin-coupled polymers are accordingly of essentially the same molecular weight. This results in such conventional tin-coupled polymers having a low polydispersity. For instance, conventional tin-coupled polymers normally having a ratio of weight average molecular weight to number average molecular weight which is within the range of about 1.01 to about 1.1
This invention is based upon the unexpected finding that greatly improved properties for tire rubbers, such as lower hysteresis, can be attained by asymmetrically coupling the rubber. For instance, such asymmetrically coupled polymers can be utilized in making tires having greatly improved rolling resistance without sacrificing other tire properties. These improved properties are due in part to better interaction and compatibility with carbon black. The asymmetrical tin coupling also normally leads to improve the cold flow characteristics of the rubbery polymer. Tin coupling in general also leads to better processability and other beneficial properties.
The asymmetrical tin-coupled rubbery polymers of this invention are comprised of a tin atom having polydiene arms covalently bonded thereto. At least one of the polydiene arms bonded to the tin atom will be a low number molecular weight arm having a number average molecular weight of less than about 40,000. It is also critical for the asymmetrical tin-coupled rubbery polymer to have at least one high molecular weight polydiene arm bonded to the tin atom. This high molecular weight arm will have a number average molecular weight which is at least 80,000. The ratio of the weight average molecular weight to the number average molecular weight of the asymmetrical tin-coupled rubbery polymers of this invention will also be within the range of about 2 to about 2.5.
This invention more specifically discloses an asymmetrical tin-coupled rubbery polymer which is particularly valuable for use in manufacturing tire tread compounds, said asymmetrical tin-coupled rubbery polymer being comprised of a tin atom having at least three polydiene arms covalently bonded thereto, wherein at least one of said polydiene arms has a number average molecular weight of less than about 40,000, wherein at least one of said polydiene arms has a number average molecular weight of at least about 80,000, and wherein the ratio of the weight average molecular weight to the number average molecular weight of the asymmetrical tin-coupled rubbery polymer is within the range of about 2 to about 2.5.
This invention also discloses an asymmetrical tin-coupled rubbery polymer which is particularly valuable for use in manufacturing tire tread compounds, said asymmetrical tin-coupled rubbery polymer being comprised of a tin atom having at least three polydiene arms covalently bonded thereto, wherein at least one of said polydiene arms has a number average molecular weight of less than about 40,000, wherein at least one of said polydiene arms has a number average molecular weight of at least about 80,000, wherein the polydiene arms do not contain blocks of more then about 4 repeat units of vinyl aromatic monomers, and wherein the ratio of the weight average molecular weight to the number average molecular weight of the asymmetrical tin-coupled rubbery polymer is within the range of about 2 to about 2.5.
This invention also reveals a process for preparing an asymmetrical tin-coupled rubbery polymer which comprises: (1) continuously polymerizing at least one diene monomer to a conversion of at least about 90 percent utilizing an anionic initiator to produce a polymer cement containing living polydiene rubber chains, wherein some of the living polydiene rubber chains are low molecular weight polydiene rubber chains having a number average molecular weight of less than about 40,000, and wherein some of the living polydiene rubber chains are high molecular weight polydiene rubber chains having a number average molecular weight of greater than about 80,000; and (2) continuously adding a tin halide to the polymer cement in a separate reaction vessel to produce the asymmetrically tin-coupled rubbery polymer, wherein said asymmetrical tin-coupled rubbery polymer has a polydispersity which is within the range of about 2 to about 2.5.
The present invention further discloses an asymmetrical tin-coupled rubbery polymer which is particularly valuable for use in manufacturing tire tread compounds, said asymmetrical tin-coupled rubbery polymer being comprised of a tin atom having at least three polydiene arms covalently bonded thereto, wherein said polydiene arms have a weight average molecular weight which is within the range of about 80,000 to about 300,000, wherein the polydiene arms do not contain blocks of more than about 4 repeat units of vinyl aromatic monomers, and wherein the ratio of the weight average molecular weight to the number average molecular weight of the asymmetrical tin-coupled rubbery polymer is within the range of about 1.4 to about 2.0.
The stability of the asymmetrical tin-coupled rubbery polymers of this invention can be improved by adding a tertiary chelating amine thereto subsequent to the time at which the tin-coupled rubbery polymer is coupled. N,N,Nxe2x80x2, Nxe2x80x2-tetramethylethylenediamine (TMEDA) is a representative example of a tertiary chelating amine which is preferred for utilization in stabilizing the polymers of this invention.
Virtually any type of rubbery polymer prepared by anionic polymerization can be asymmetrically tin-coupled in accordance with this invention. The rubbery polymers that can be asymmetrically coupled will typically be synthesized by a solution polymerization technique utilizing an organolithium compound as the initiator. These rubbery polymers will accordingly normally contain a xe2x80x9clivingxe2x80x9d lithium chain end.
The polymerizations employed in synthesizing the living rubbery polymers will normally be carried out in a hydrocarbon solvent which can be one or more aromatic, paraffinic or cycloparaffinic compounds. These solvents will normally contain from 4 to 10 carbon atoms per molecule and will be liquid under the conditions of the polymerization. Some representative examples of suitable organic solvents include pentane, isooctane, cyclohexane, methylcyclohexane, isohexane, n-heptane, n-octane, n-hexane, benzene, toluene, xylene, ethylbenzene, diethylbenzene, isobutylbenzene, petroleum ether, kerosene, petroleum spirits, petroleum naphtha, and the like, alone or in admixture.
In the solution polymerization, there will normally be from 5 to 30 weight percent monomers in the polymerization medium. Such polymerization media are, of course, comprised of the organic solvent and monomers. In most cases, it will be preferred for the polymerization medium to contain from 10 to 25 weight percent monomers. It is generally more preferred for the polymerization medium to contain 15 to 20 weight percent monomers.
The rubbery polymers which are asymmetrically coupled in accordance with this invention can be made by the homopolymerization of a conjugated diolefin monomer or by the copolymerization of a conjugated diolefin monomer with a vinyl aromatic monomer. It is, of course, also possible to make living rubbery polymers which can be asymmetrically tin-coupled by polymerizing a mixture of conjugated diolefin monomers with one or more ethylenically unsaturated monomers, such as vinyl aromatic monomers. The conjugated diolefin monomers which can be utilized in the synthesis of rubbery polymers which can be asymmetrically tin-coupled in accordance with this invention generally contain from 4 to 12 carbon atoms. Those containing from 4 to 8 carbon atoms are generally preferred for commercial purposes. For similar reasons, 1,3-butadiene and isoprene are the most commonly utilized conjugated diolefin monomers. Some additional conjugated diolefin monomers that can be utilized include 2,3-dimethyl-1,3-butadiene, piperylene, 3-butyl-1,3-octadiene, 2-phenyl-1,3-butadiene, and the like, alone or in admixture.
Some representative examples of ethylenically unsaturated monomers that can potentially be synthesized into rubbery polymers which can be asymmetrically tin-coupled in accordance with this invention include alkyl acrylates, such as methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate and the like; vinylidene monomers having one or more terminal CH2xe2x95x90CHxe2x80x94 groups; vinyl aromatics such as styrene, xcex1-methylstyrene, bromostyrene, chlorostyrene, fluorostyrene and the like; xcex1-olefins such as ethylene, propylene, 1-butene and the like; vinyl halides, such as vinylbromide, chloroethane (vinylchloride), vinylfluoride, vinyliodide, 1,2-dibromoethene, 1,1-dichloroethene (vinylidene chloride), 1,2-dichloroethene and the like; vinyl esters, such as vinyl acetate; xcex1, xcex2-olefinically unsaturated nitrites, such as acrylonitrile and methacrylonitrile; xcex1, xcex2-olefinically unsaturated amides, such as acrylamide, N-methyl acrylamide, N,N-dimethylacrylamide, methacrylamide and the like.
Rubbery polymers which are copolymers of one or more diene monomers with one or more other ethylenically unsaturated monomers will normally contain from about 50 weight percent to about 99 weight percent conjugated diolefin monomers and from about 1 weight percent to about 50 weight percent of the other ethylenically unsaturated monomers in addition to the conjugated diolefin monomers. For example, copolymers of conjugated diolefin monomers with vinylaromatic monomers, such as styrene-butadiene rubbers which contain from 50 to 99 weight percent conjugated diolefin monomers and from 1 to 50 weight percent vinylaromatic monomers, are useful in many applications.
Vinyl aromatic monomers are probably the most important group of ethylenically unsaturated monomers which are commonly incorporated into polydienes. Such vinyl aromatic monomers are, of course, selected so as to be copolymerizable with the conjugated diolefin monomers being utilized. Generally, any vinyl aromatic monomer which is known to polymerize with organolithium initiators can be used. Such vinyl aromatic monomers typically contain from 8 to 20 carbon atoms. Usually, the vinyl aromatic monomer will contain from 8 to 14 carbon atoms. The most widely used vinyl aromatic monomer is styrene. Some examples of vinyl aromatic monomers that can be utilized include styrene, 1-vinylnaphthalene, 2-vinylnaphthalene, xcex1-methylstyrene, 4-phenylstyrene, 3-methylstyrene and the like. In most cases the rubber will contain no more than about 25 weight percent of the vinylaromatic monomer. It is critical for such vinyl aromatic monomers to be distributed randomly throughout the polymer chains of such rubbery polymers. Thus, a modifier such as sodium dodecylbenzene sulfonate will be employed during the polymerization to insure that repeat units which are derived from the vinyl aromatic monomers are randomly distributed throughout the polymer chains.
Some representative examples of rubbery polymers which can be asymmetrically tin-coupled in accordance with this invention include polybutadiene, polyisoprene, random styrene-butadiene rubber (SBR), random xcex1-methylstyrene-butadiene rubber, random xcex1-methylstyrene-isoprene rubber, random styrene-isoprene-butadiene rubber (SIBR), random styrene-isoprene rubber (SIR), random isoprene-butadiene rubber (IBR), random xcex1-methylstyrene-isoprene-butadiene rubber and random xcex1-methylstyrene-styrene-isoprene-butadiene rubber.
The polymerizations employed in making the rubbery polymer are typically initiated by adding an organolithium initiator to an organic polymerization medium which contains the monomers. Such polymerizations are typically carried out utilizing continuous polymerization techniques. In such continuous polymerizations, monomers and initiator are continuously added to the organic polymerization medium with the rubbery polymer synthesized being continuously withdrawn. Such continuous polymerizations are typically conducted in a multiple reactor system.
The organolithium initiators which can be employed in synthesizing rubbery polymers which can be asymmetrically coupled in accordance with this invention include the monofunctional and multifunctional types known for polymerizing the monomers described herein. The multifunctional organolithium initiators can be either specific organolithium compounds-or can be multifunctional types which are not necessarily specific compounds but rather represent reproducible compositions of regulable functionality.
The amount of organolithium initiator utilized will vary with the monomers being polymerized and with the molecular weight that is desired for the polymer being synthesized. However, as a general rule, from 0.01 to 1 phm (parts per 100 parts by weight of monomer) of an organolithium initiator will be utilized. In most cases, from 0.01 to 0.1 phm of an organolithium initiator will be utilized with it being preferred to utilize 0.025 to 0.07 phm of the organolithium initiator.
The choice of initiator can be governed by the degree of branching and the degree of elasticity desired for the polymer, the nature of the feedstock and the like. With regard to the feedstock employed as the source of conjugated diene, for example, the multifunctional initiator types generally are preferred when a low concentration diene stream is at least a portion of the feedstock, since some components present in the unpurified low concentration diene stream may tend to react with carbon lithium bonds to deactivate initiator activity, thus necessitating the presence of sufficient lithium functionality in the initiator so as to override such effects.
The multifunctional initiators which can be used include those prepared by reacting an organomonolithium compounded with a multivinylphosphine or with a multivinylsilane, such a reaction preferably being conducted in an inert diluent such as a hydrocarbon or a mixture of a hydrocarbon and a polar organic compound. The reaction between the multivinylsilane or multivinylphosphine and the organomonolithium compound can result in a precipitate which can be solubilized, if desired, by adding a solubilizing monomer such as a conjugated diene or monovinyl aromatic compound, after reaction of the primary components. Alternatively, the reaction can be conducted in the presence of a minor amount of the solubilizing monomer. The relative amounts of the organomonolithium compound and the multivinylsilane or the multivinylphosphine preferably should be in the range of about 0.33 to 4 moles of organomonolithium compound per mole of vinyl groups present in the multivinylsilane or multivinylphosphine employed. It should be noted that such multifunctional initiators are commonly used as mixtures of compounds rather than as specific individual compounds.
Exemplary organomonolithium compounds include ethyllithium, isopropyllithium, n-butyllithium, sec-butyllithium, tert-octyllithium, n-eicosyllithium, phenyllithium, 2-naphthyllithium, 4-butylphenyllithium, 4-tolyllithium, 4-phenylbutyllithium, cyclohexyllithium and the like.
Exemplary multivinylsilane compounds include tetravinylsilane, methyltrivinylsilane, diethyldivinylsilane, di-n-dodecyldivinylsilane, cyclohexyltrivinylsilane, phenyltrivinylsilane, benzyltrivinylsilane, (3-ethylcyclohexyl) (3-n-butylphenyl)divinylsilane and the like.
Exemplary multivinylphosphine compounds include trivinylphosphine, methyldivinylphosphine, dodecyldivinylphosphine, phenyldivinylphosphine, cyclooctyldivinylphosphine and the like.
Other multifunctional polymerization initiators can be prepared by utilizing an organomonolithium compound, further together with a multivinylaromatic compound and either a conjugated diene or monovinylaromatic compound or both. These ingredients can be charged initially, usually in the presence of a hydrocarbon or a mixture of a hydrocarbon and a polar organic compound as a diluent. Alternatively, a multifunctional polymerization initiator can be prepared in a two-step process by reacting the organomonolithium compound with a conjugated diene or monovinyl aromatic compound additive and then adding the multivinyl aromatic compound. Any of the conjugated dienes or monovinyl aromatic compounds described can be employed. The ratio of conjugated diene or monovinyl aromatic compound additive employed preferably should be in the range of about 2 to 15 moles of polymerizable compound per mole of organolithium compound. The amount of multivinylaromatic compound employed preferably should be in the range of about 0.05 to 2 moles per mole of organomonolithium compound.
Exemplary multivinyl aromatic compounds include 1,2-divinylbenzene, 1,3-divinylbenzene, 1,4-divinylbenzene, 1,2,4-trivinylbenzene, 1,3-divinylnaphthalene, 1,8-divinylnaphthalene, 1,3,5-trivinylnaphthalene, 2,4-divinylbiphenyl, 3,5,4xe2x80x2-trivinylbiphenyl, m-diisopropenyl benzene, p-diisopropenyl benzene, 1,3-divinyl-4,5,8-tributylnaphthalene and the like. Divinyl aromatic hydrocarbons containing up to 18 carbon atoms per molecule are preferred, particularly divinylbenzene as either the ortho, meta or para isomer and commercial divinylbenzene, which is a mixture of the three isomers, and other compounds, such as the ethylstyrenes, also is quite satisfactory.
Other types of multifunctional initiators can be employed such as those prepared by contacting a sec-or tert-organomonolithium compound with 1,3-butadiene, at a ratio of about 2 to 4 moles of the organomonolithium compound per mole of the 1,3-butadiene, in the absence of added polar material in this instance, with the contacting preferably being conducted in an inert hydrocarbon diluent, though contacting without the diluent can be employed if desired.
Alternatively, specific organolithium compounds can be employed as initiators, if desired, in the preparation of polymers in accordance with the present invention. These can be represented by R(Li)x wherein R represents a hydrocarbyl radical containing from 1 to 20 carbon atoms, and wherein x is an integer of 1 to 4. Exemplary organolithium compounds are methyllithium, isopropyllithium, n-butyllithium, sec-butyllithium, tert-octyllithium, n-decyllithium, phenyllithium, 1-naphthyllithium, 4-butylphenyllithium, p-tolyllithium, 4-phenylbutyllithium, cyclohexyllithium, 4-butylcyclohexyllithium, 4-cyclohexylbutyllithium, dilithiomethane, 1,4-dilithiobutane, 1,10-dilithiodecane, 1,20-dilithioeicosane, 1,4-dilithiocyclohexane, 1,4-dilithio-2-butane, 1,8-dilithio-3-decene, 1,2-dilithio-1,8-diphenyloctane, 1,4-dilithiobenzene, 1,4-dilithionaphthalene, 9,10-dilithioanthracene, 1,2-dilithio-1,2-diphenylethane, 1,3,5-trilithiopentane, 1,5,15-trilithioeicosane, 1,3,5-trilithiocyclohexane, 1,3,5,8-tetralithiodecane, 1,5,10,20-tetralithioeicosane, 1,2,4,6-tetralithiocyclohexane, 4,4xe2x80x2-dilithiobiphenyl and the like. 
The polymerization temperature utilized can vary over a broad range of from about xe2x88x9220xc2x0 C. to about 180xc2x0 C. In most cases, a temperature within the range of about 30xc2x0 C. to about 125xc2x0 C. will be utilized. It is typically most preferred for the polymerization temperature to be within the range of about 60xc2x0 C. to about 85xc2x0 C. The pressure used will normally be sufficient to maintain a substantially liquid phase under the conditions of the polymerization reaction.
The polymerization is conducted for a length of time sufficient to permit substantially complete polymerization of monomers. In other words, the polymerization is normally carried out until high conversions are attained. The polymerization is then terminated by the continuous addition of a tin coupling agent. This continuous addition of tin coupling agent is normally done in a reaction zone separate from the zone where the bulk of the polymerization is occurring. In other words, the tin coupling agent will typically be added only after a high degree of conversion has already been attained. For instance, the tin coupling agent will normally be added only after a monomer conversion of greater than about 90 percent has been realized. It will typically be preferred for the monomer conversion to reach at least about 95 percent before the tin coupling agent is added. As a general rule, it is most preferred for the monomer conversion to exceed about 98 percent before the tin coupling agent is added. The tin coupling agent will normally be added in a separate reaction vessel after the desired degree of conversion has been attained. The tin coupling agent can be added in a hydrocarbon solution; e.g., in cyclohexane, to the polymerization admixture with suitable mixing for distribution and reaction.
The tin coupling agent will normally be a tin tetrahalide, such as tin tetrachloride, tin tetrabromide, tin tetrafluoride or tin tetraiodide. However, tin trihalides can also optionally be used. In cases where tin trihalides are utilized, a coupled polymer having a maximum of three arms results. To induce a higher level of branching, tin tetrahalides are normally preferred. As a general rule, tin tetrachloride is most preferred.
Broadly, and exemplary, a range of about 0.01 to 4.5 milliequivalents of tin coupling agent is employed per 100 grams of the rubbery polymer. It is normally preferred to utilize about 0.01 to about 1.5 milliequivalents of the tin coupling agent per 100 grams of polymer to obtain the desired Mooney viscosity. The larger quantities tend to result in production of polymers containing terminally reactive groups or insufficient coupling. One equivalent of tin coupling agent per equivalent of lithium is considered an optimum amount for maximum branching. For instance, if a tin tetrahalide is used as the coupling agent, one mole of the tin tetrahalide would be utilized per four moles of live lithium ends. In cases where a tin trihalide is used as the coupling agent, one mole of the tin trihalide will optimally be utilized for every three moles of live lithium ends. The tin coupling agent can be added in a hydrocarbon solution, e.g., in cyclohexane, to the polymerization admixture in the reactor with suitable mixing for distribution and reaction.
After the tin coupling has been completed, a tertiary chelating alkyl 1,2-ethylene diamine can optionally be added to the polymer cement to stabilize the asymmetrically tin-coupled rubbery polymer. The tertiary chelating amines which can be used are normally chelating alkyl diamines of the structural formula: 
wherein n represents an integer from 1 to about 6, wherein A represents an alkylene group containing from 1 to about 6 carbon atoms and wherein R1, R2, R3 and R4 can be the same or different and represent alkyl groups containing from 1 to about 6 carbon atoms. The alkylene group A is the formula xe2x80x94(xe2x80x94CH2xe2x80x94)m wherein m is an integer from 1 to about 6. The alkylene group will typically contain from 1 to 4 carbon atoms (m will be 1 to 4) and will preferably contain 2 carbon atoms. In most cases, n will be an integer from 1 to about 3 with it being preferred for n to be 1. It is preferred for R1, R2, R3 and R4 to represent alkyl groups which contain from 1 to 3 carbon atoms. In most cases, R1, R2, R3 and R4 will represent methyl groups.
A sufficient amount of the chelating amine should be added to complex with any residual tin coupling agent remaining after completion of the coupling reaction.
In most cases, from about 0.01 phr (parts by weight per 100 parts by weight of dry rubber) to about 2 phr of the chelating alkyl 1,2-ethylene diamine will be added to the polymer cement to stabilize the rubbery polymer. Typically, from about 0.05 phr to about 1 phr of the chelating alkyl 1,2-ethylene diamine will be added. More typically, from about 0.1 phr to about 0.6 phr of the chelating alkyl 1,2-ethylene diamine will be added to the polymer cement to stabilize the rubbery polymer.
After the polymerization, asymmetrical tin coupling, and optionally the stabilization step, has been completed, the asymmetrical tin-coupled rubbery polymer can be recovered from the organic solvent. The asymmetrical tin-coupled rubbery polymer can be recovered from the organic solvent and residue by means such as decantation, filtration, centrification and the like. It is often desirable to precipitate the asymmetrically tin-coupled rubbery polymer from the organic solvent by the addition of lower alcohols containing from about 1 to about 4 carbon atoms to the polymer solution. Suitable lower alcohols for precipitation of the rubber from the polymer cement include methanol, ethanol, isopropyl alcohol, normal-propyl alcohol and t-butyl alcohol. The utilization of lower alcohols to precipitate the asymmetrically tin-coupled rubbery polymer from the polymer cement also xe2x80x9ckillsxe2x80x9d any remaining living polymer by inactivating lithium end groups. After the asymmetrically tin-coupled rubbery polymer is recovered from the solution, steam-stripping can be employed to reduce the level of volatile organic compounds in the asymmetrically tin-coupled rubbery polymer.
The asymmetrical tin-coupled rubbery polymers of this invention are comprised of a tin atom having at least three polydiene arms covalently bonded thereto. At least one of the polydiene arms bonded to the tin atom has a number average molecular weight of less than about 40,000 and at least one of the polydiene arms bonded to the tin atom has a number average molecular weight of at least about 80,000. The ratio of the weight average molecular weight to the number average molecular weight of the asymmetrical tin-coupled rubbery polymer will also be within the range of about 1.4 to about 2.5.
The asymmetrical tin-coupled rubbery polymers of this invention are of the structural formula: 
wherein R1, R2, R3 and R4 can be the same or different and are selected from the group consisting of alkyl groups and polydiene arms (polydiene rubber chains), with the proviso that at least three members selected from the group consisting of R1, R2, R3 and R4 are polydiene arms, with the proviso that at least one member selected from the group consisting of R1, R2, R3 and R4 is a low molecular weight polydiene arm having a number average molecular weight of less than about 40,000, with the proviso that at least one member selected from the group consisting of R1, R2, R3 and R4 is a high molecular weight polydiene arm having a number average molecular weight of greater than about 80,000, and with the proviso that the ratio of the weight average molecular weight to the number average molecular weight of the asymmetrical tin-coupled rubbery polymer is within the range of about 1.4 to about 2.5. It should be noted that R1, R2, R3 and R4 can be alkyl groups because it is possible for the tin halide coupling agent to react directly with alkyl lithium compounds which are used as the polymerization initiator.
In most cases, four polydiene arms will be covalently bonded to the tin atom in the asymmetrical tin-coupled rubbery polymer. In such cases, R1, R2, R3 and R4 will all be polydiene arms. The asymmetrical tin-coupled rubbery polymer will often contain a polydiene arm of intermediate molecular weight as well as the low molecular weight arm and the high molecular weight arm. Such intermediate molecular weight arms will have a molecular weight that is within the range of about 45,000 to about 75,000. It is normally preferred for the low molecular polydiene arm to have a molecular weight of less than about 30,000 with it being most preferred for the low molecular weight arm to have a molecular weight of less than about 25,000. It is normally preferred for the high molecular polydiene arm to have a molecular weight of greater than about 90,000 with it being most preferred for the high molecular weight arm to have a molecular weight of greater than about 100,000.
The polydiene arms in the asymmetrical tin-coupled rubbery polymers of this invention will typically have a weight average molecular weight that is within the range of about 80,000 to about 300,000. The polydiene arms will more typically have a weight average molecular weight that is within the range of about 120,000 to about 250,000. The ratio of the weight average molecular weight to the number average molecular weight of the asymmetrical tin-coupled rubbery polymer is typically within the range of about 1.4 to about 2.5. The ratio of the weight average molecular weight to the number average molecular weight of the asymmetrical tin-coupled rubbery polymer is typically within the range of about 1.4 to about 2.0. The ratio of the weight average molecular weight to the number average molecular weight of the asymmetrical tin-coupled rubbery polymer is more typically within the range of about 1.45 to about 1.8. The ratio of the weight average molecular weight to the number average molecular weight of the asymmetrical tin-coupled rubbery polymer is most typically within the range of about 1.5 to about 2.7. Normally at least one of the polydiene arms covalently bonded to the tin has a weight average molecular weight of less than about 150,000 and at least one of the polydiene arms covalently bonded to the tin atom has a weight average molecular weight of greater than about 200,000.
The polydiene arms in the asymmetrical tin-coupled rubbery polymers of this invention are not block copolymers. In cases where vinyl aromatic monomers, such as styrene, are present in the polydiene arms they are not present in blocks containing more than about 4 repeat units. In other words, the vinyl aromatic monomers will be distributed in a random fashion throughout the polydiene arms.